PROJECT SUMMARY All organisms must simultaneously balance multiple competing demands on their energetic and physiological resources. This can result in conflict among traits, driving physiological and evolutionary tradeoffs. Defense against infection and reproductive capacity are each under strong natural selection as important contributors to evolutionary fitness, both are energetically demanding, and tradeoffs between the two are nearly universal in plants and animals, including those with public health importance. Yet despite their ubiquity and importance, surprisingly little is known about the mechanistic basis for reproduction-immunity tradeoffs. This shortcoming in knowledge limits inference about the evolutionary process and applies practical limitations on the capacity for interventions to control insect pests in agricultural or public health contexts. The proposed project will use the tractable model insect Drosophila melanogaster to determine mechanisms by which reproductive investment constrains immune performance, exploring the evolutionary implications of the tradeoff and providing rare understanding of how life history tradeoffs arise and are maintained. The project will test of the hormonal theory of pleiotropy, which posits that endocrine signaling can act as a master regulator to shape life history tradeoffs, with a focus on insect Juvenile Hormone (JH). Initial experiments will determine the temporal kinetics of JH signaling in response to mating and the quantitative relationship between JH signal intensity and degree of immune suppression. Subsequent experiments will determine the mechanisms by which JH suppresses immunity, seeking to identify novel regulatory processes, testing a hypothesis that the tradeoff can arise through strain on cellular infrastructure, and determining whether immunosuppression can be decoupled from reproductive investment. Additional experiments will test whether naturally occurring genetic variation in JH signaling determines natural phenotypic variability in the magnitude of the reproduction-immunity tradeoff and whether energetic investment required for reproductive output directly limits the quality of immune defense. The data obtained will comprise an in-depth study of the focal system and will reveal principles that can be generalized across diverse systems.